Looking at the diagram, the network address under 'Admin' was blurred out. So assuming that is 172.16.3.96 then yes "C" would be a good answer for Network A.

For Link A I would choose answer "D" and the reason is what they are really asking is to assign addresses to Link A and Network A that will not overlap with other networks (because each router interface is a network boundary). It is also assumed they would want to have the most efficient use of the address space.

Answer "D" will use the address 0-3 that is currently not being used. Also because it is a link, you will only need two addresses for each interface on the router. A /30 network does just that, 0 will be the network address, .3 is the broadcast address, .1 and .2 will be the addresses for your router interfaces.

Thanks for your reply, I understand what you mean, yes you are right, the 2nd choice of correct answer is "D" but why "D"? Why not "E"? Even the IP address shown in answer "E" has 30 bits of Subneting "/30".

For Link A the correct answer is D because is a point to point which you only need 2 IPs usable the /30 link. you dont use E because that IPs can be used in a larger subnet. to better understand break the network in smaller pieces starting from the end.

You have 172.16.3.0/24 break these network into to /25 networks 172.16.3.0/25 and 172.16.3.128/25 (these will be used for Network A so is gone)

then break the 172.16.3.0/25 into smaller and these i believe will help you understand the subnetting